The present invention relates to a device for correcting the recording density of a recording device having a thermal head implemented as an array of a plurality of elements.
A recording density correcting device for the above application is disclosed in, for example, Japanese Patent Laid-Open Publication No. 201876/1984. The device disclosed in this prior art has resistance measuring means for measuring the resistance of each element of the head beforehand, and heat control means for controlling heat to be generated on the basis of the resistance. Specifically, paying attention to irregularities in the resistance of the element, the device controls a current so as to cause all of the elements to generate the same amount of heat. Further, since the device measures the resistance element by element at a predetermined time, it allows all the elements to generate uniform heat despite the resistance which varies due to aging. Another type of recording density correcting device proposed in the past measures print sample densities and controls head in matching relation to the element-by-element density.
However, the problem with the device taught in the above Laid-Open Publication is that the thickness of a glaze layer provided on the head is not uniform throughout the elements. In actual printing, even if the current is so controlled as to cause the elements to generate the same amount of heat, the thermal efficiency for coloring differs from one element to another due to the irregular thickness distribution of the glaze layer. Hence, the resulting print density is not always uniform. Stated another way, since the print density noticeably changes depending on the thickness of the glaze layer, simply correcting the irregularities in the resistance of the element cannot provide a solid image with satisfactory uniform density.
While the above problem will, of course, be eliminated if a thermal head whose glaze layer has a substantially uniform thickness is used, such a head is not practical from the yield standpoint, i.e., the cost standpoint.
The conventional device of the type measuring the print sample density promotes accurate correction since the print sample density suffered from the irregular thickness of the glaze layer is measured and then fed back to the print density. However, since the resistance of each element changes due to aging, correction data have to be sampled at a predetermined period.